Function observeVisibleRects

packages/react-reconciler/src/ReactTestSelectors.js:559–608

(
  hostRoot: Instance,
  selectors: Array<Selector>,
  callback: (intersections: Array<{ratio: number, rect: BoundingRect}>) => void,
  options?: IntersectionObserverOptions,
)

) => void;

export function observeVisibleRects(
  hostRoot: Instance,
  selectors: Array<Selector>,
  callback: (intersections: Array<{ratio: number, rect: BoundingRect}>) => void,
  options?: IntersectionObserverOptions,
): {disconnect: () => void} {
  if (!supportsTestSelectors) {
    throw new Error('Test selector API is not supported by this renderer.');
  }

  const instanceRoots = findAllNodes(hostRoot, selectors);

  const {disconnect, observe, unobserve} = setupIntersectionObserver(
    instanceRoots,
    callback,
    options,
  );

  // When React mutates the host environment, we may need to change what we're listening to.
  const commitHook = () => {
    const nextInstanceRoots = findAllNodes(hostRoot, selectors);

    instanceRoots.forEach(target => {
      if (nextInstanceRoots.indexOf(target) < 0) {
        unobserve(target);
      }
    });

    nextInstanceRoots.forEach(target => {
      if (instanceRoots.indexOf(target) < 0) {
        observe(target);
      }
    });
  };

  commitHooks.push(commitHook);

  return {
    disconnect: () => {
      // Stop listening for React mutations:
      const index = commitHooks.indexOf(commitHook);
      if (index >= 0) {
        commitHooks.splice(index, 1);
      }

      // Disconnect the host observer:
      disconnect();
    },
  };
}